Complete security for any amplifier.

[Vermes]

The circuit is a complete security for a power amplifiers and speakers. This device eliminates knocking on power on/off, secures the power amplifier against overheating (it cuts off the speakers when the temperature is over 65 degrees of the radiator), secures the speaker against DC power on the amplifier output (when it detects the power amplifiers error, it cuts off the speakers).
Elements needed:

• R1, R2 – 220Ohm 1/4W
• R3 - 33k 1/4W
• R4 - 22k 1/4W
• R5 - 150Ohm 1/2W (when overheat 1W)
• R6 - NTC Thermistor 4.7k in room temperature (22 degrees)
• R7, R8 6,8k 1/4W
• C1, C4 - 4,7uF 63V
• C2, C3 - 100uF 25V
• D1 to D6 - 1N4148
• D7 – Zener's Diode 24V 1/2W (use better 1W)
• D8, D9 - 1N4007
• Q1 to Q3 BC547C
• K1 – 24V relay with two pairs of contacts with load from 2A(20W) to 5A (over 80W)

Attention: The presented circuit is without Zener Diode D7 and R5 resistor. Instead of this elements, a LM7824 was used (it simplified the circuit).
Attention!!!: There is a mistake on the schema! Between D2 and D5, there should be a third diode to make series of 3. It should be the same type as D2 and D5.
To adjust the safety temperature, just use a thermistor with another resistance (lower to have a lower threshold) or add a potentiometer.
To adjust the time to switch on the speakers, change the C1 capacitor from 4.7uF to 47uF (bigger capacity - longer time).

The schema with Zener's Diode:



Schema to PCB with LM7824:



The final result:



Mounted version (in WS354):



Link to original thread - Komplet zabezpieczeń do każdego wzmacniacza